1. Field of the Invention
The present invention relates to a levitation type magnetic head assembly and a magnetic disk apparatus using the magnetic head assembly, and more particularly a magnetic head assembly having high reliability with a less variation in a crown of a magnetic head slider due to an ambient temperature variation and a magnetic disk apparatus using this magnetic head assembly.
2. Description of the Related Art
Recently, particular attention has been paid to a small-sized magnetic disk apparatus having a disk of a diameter of, e.g. 2.5 inches or less, which can be used as a data storage device for a small-sized personal computer of a lap top type or a notebook type. With this trend, there is a greater demand for a magnetic disk apparatus with a magnetic disk of a large capacity and a structure capable of withstanding the external ambience such as temperature, humidity, etc.
According to an improved method of increasing the capacity of a magnetic disk, the amount of levitation of a magnetic head slider from a disk surface is decreased, thereby increasing a linear recording density or a recording density of a disk in a circumferential direction. For example, in a currently available magnetic disk apparatus, the amount of levitation is 0.1 .mu.m or less at normal temperature.
According to another method, the performance of a magnetic head itself is improved. Such improved-performance heads include a composite head wherein a recording/reproducing transducer of ferrite is buried in a ceramic slider of TiCaO (calcium titanate); a thin film head wherein a recording/reproducing transducer is formed on a ceramic slider of Al.sub.2 O.sub.3 TiC (aluminum titanium carbide), etc. by using a thin film forming process; and a magnetoresistive thin film composite head.
In the above method of increasing the recording density by decreasing the amount of levitation of the magnetic head slider, however, there is a problem. In general, thermal expansion, etc. occur in the structural elements of a magnetic disk apparatus due to a variation in ambient temperatures. In particular, a difference in linear expansion coefficient among the structural elements (a slider member and a flexure spring) results in a change in shape of the magnetic head slider including a portion facing the disk. Such a change in shape of the magnetic head slider adversely affects the amount of levitation of the magnetic head slider while the magnetic disk apparatus is being operated. Consequently, the reliability of vibration recording/reproduction is considerably deteriorated. Specifically, if the amount of levitation of the magnetic head slider increases, a read signal amplitude in a data recording/reproducing mode lowers relatively. On the other hand, if the amount of levitation of the magnetic head slider decreases, the magnetic head slider may come into contact with a recording medium or a disk, in particular, with a projection on a disk if such a projection is provided. If the magnetic head slider comes into contact with the disk, data may be destroyed.
This problem will now be explained in greater detail with reference to a magnetic head assembly of a conventional magnetic disk apparatus.
FIG. 1 is a perspective view of a magnetic assembly applied to a conventional magnetic disk apparatus, FIG. 2A is a plan view of that side of the magnetic head assembly, which faces the disk, FIG. 2B is a side view of the magnetic head assembly, and FIG. 2C is a plan view of the opposite side of the magnetic head assembly.
A suspension 1 is fixed to a distal end portion of an arm 2 of an actuator (not shown). The suspension 1 comprises a magnetic head slider 3, a flexure spring 4, which is also called "gimbal", a load spring 5, and a mount support portion 6.
The magnetic head slider 3 is bonded to a slider fixing portion 4a of the flexure spring 4 by means of an epoxy adhesive, etc. The flexure spring 4 is provided with a slit having a U-shape in a plan view. An inner part of the flexure spring 4, surrounded by the U-shaped slit, serves as the slider fixing portion 4a. A part of the flexure spring 4 is welded to one end portion of the load spring 5 having a higher rigidity (or a greater thickness) than the flexure spring 4. The other end portion of the load spring 5 is provided with the mount support portion 6 having a higher rigidity (or a greater thickness) than the load spring 5. The entire suspension 1 is fixed to the arm 2 of the actuator via the mount support portion 6.
On the other hand, the slider fixing portion 4a of the flexure spring 4 is provided with a hemispherical projection (pivot) 4b. A top portion of the projection 4b is put in contact with the surface of the load spring 5. Thereby, the load spring 5 absorbs a levitation force of the magnetic head slider 3 due to an air bearing effect caused by a relative speed between the magnetic head slider 3 and a disk D.
The magnetic head slider 3 is bonded to the slider fixing portion 4a of the flexure spring 4 over the entire mutually contacting surfaces by an epoxy adhesive, etc. The thickness of a layer of the adhesive is set at about 20 .mu.m or less by controlling the amount of the adhesive supplied in the manufacturing process.
FIG. 3 is a perspective view of the magnetic head slider 3. The body of the magnetic head slider 3 is made of, e.g. a ceramic material such as AlTiC. A pair of slider rails 7a and 7b are provided on that surface of the magnetic head slider 3, which faces the disk D, along an axis of symmetry of the load spring 5. In addition, those surfaces of the slider rails 7a and 7b of the magnetic head slider 3 on the air entrance side, which are opposed to the disk D, are provided with tapers 7c and 7d for levitating the magnetic head slider 3 in a sub-micron unit by the air bearing effect caused by the relative speed between the magnetic head slider 3 and the disk D. A transducer 8 for performing signal recording/reproduction is provided on the air exit side of the magnetic head slider 3. It should be noted that the magnetic head slider 3 is designed such that when the magnetic head slider 3 is levitated, a levitation amount a of the taper side of the slider 3 is always greater than a levitation amount b of the transducer side thereof, as shown in FIG. 4.
In general, a certified range of temperatures for the operation of a magnetic disk apparatus is 0.degree. C. to 50.degree. C. FIGS. 5A and 5B show analysis results, obtained by a structural analysis program, on the change of shape of the magnetic head slider 3 due to a difference in linear expansion coefficient between the slider material and the flexure spring material in this range of temperatures.
Suppose that a temperature for normal use of the magnetic disk apparatus is 25.degree. C., a relative high temperature is +25.degree. C. (50.degree.C.), and a relative low temperature is -25.degree. (0.degree.C.). The length L, height H and width Ws of the magnetic head slider 3, as shown in FIGS. 2B and 2C, are set as follows:
L=2.5 mm,
H=0.5 mm, and
Ws=2.0 mm.
The thickness t and width Wf of the flexure spring 4 are set as follows:
t=0.03 mm, and
Wf=0.9 mm.
The linear expansion coefficients of the magnetic head slider 3 and flexure spring 4 are respectively set at 7.85.times.10-6/.degree.C. and 17.5.times.10.sup.-6 /.degree.C. The magnetic head slider 3 is bonded to the slider fixing portion 4a of the flexure spring 4 over the entire mutually facing surfaces.
FIG. 5A shows the deformation state of the magnetic head slider 3 at high temperature, and FIG. 5B shows the deformation state of the same at low temperature. At high temperature, the slider rails of the magnetic head slider 3 are curved upwards, and at low temperature the slider rails are curved downwards.
FIG. 6 is a graph showing the deformation states of the slider rails of the magnetic head slider 3. The curve of the slider rail is generally called "crown."As is shown in FIG. 6, the slider rail surface deforms about 48 nm at a maximum value due to a temperature variation of .+-.25.degree. C.
The cause of deformation of the magnetic head slider 3 will now be described with reference to FIG. 7. FIG. 7 illustrates a main stress acting in the magnetic head slider 3 in the longitudinal direction (i.e. the direction of extension of the slider rail) at a high relative temperature of +25.degree.C. In FIG. 7, contour lines represent the magnitude of stress. The linear expansion coefficient of SUS 304 (JIS standard) or the material of the flexure spring 4 is 17.5.times.10.sup.-6 /.degree.C. , and the linear expansion coefficient of Al.sub.2 O.sub.3 TiC or the material of the magnetic head slider 3 is 7.85.times.10.sup.-6 /.degree.C. Accordingly, at high temperatures, the flexure spring 4 expands more than the magnetic head slider 3. A positive stress (expansion) acts on that surface of the magnetic head slider 3, which is bonded to the flexure spring 4, and a negative stress (compression) acts on the opposite-side surface. As a result, the magnetic head slider 3 deforms, as shown in FIG. 5A. At low temperatures, inversely, a negative stress (compression) acts on that surface of the magnetic head slider 3, which is bonded to the flexure spring 4, and a positive stress (expansion) acts on the opposite-side surface. As a result, the magnetic head slider 3 deforms, as shown in FIG. 5B.
The relationship between the amount of the crown and the amount of levitation will now be described with reference to FIG. 8. FIG. 8 is a graph showing the result obtained by levitation analysis on the basis of a corrected Reynolds formula, with respect to a magnetic head assembly wherein the amount of levitation of magnetic head slider 3 at normal temperature is 0.08 .mu.m (80 nm). As has been described above, when the ambient temperature is high, the magnetic head slider 3 deforms, as shown in FIG. 5A, and the amount of levitation decreases. When the ambient temperature is low, the magnetic head slider 3 deforms, as shown in FIG. 5B, and the amount of levitation increases. Specifically, as shown in FIG. 8, if the amount of crown lowers below that at the normal temperature (i.e. at the time of high temperature), the amount of levitation decreases proportionally; and if the amount of crown increases above that at the normal temperature (i.e. at the time of low temperature), the amount of levitation increases proportionally. More specifically, the amount of levitation varies in the range of .+-. about 13 nm in relation to the variation of the amount of crown in the range of .+-.20 nm.
In this manner, the amount of crown varies in accordance with the ambient temperature, and the amount of levitation varies in accordance with the variation of the amount of crown. The prevention of the variation in the amount of crown due to the variation in ambient temperature is, therefore, an important factor in obtaining a highly reliable magnetic disk apparatus.
As has been described above, owing to the variation in the amount of levitation of the magnetic head slider 3, the magnetic head slider 3 may come into contact with, or collide with, the disk D. According to the modern disk manufacturing technology, a maximum projection height h (ground height) of the disk D is about 50 nm, as shown in FIG. 9. Supposing that the amount H of levitation of the magnetic head slider 3 is 0.08 .mu.m (80 nm), a margin for avoiding collision is considered to be 30 nm. In order to avoid collision between the magnetic head slider 3 and disk D, it is therefore necessary to reduce to 30 nm or less the variation in the amount of levitation caused by the variation in the amount of crown due to temperature variation.
The above-described conventional magnetic head assembly, however, does not necessarily meet the above conditions, as will be explained below.
The variation a in the amount of levitation in relation to the variation in the amount of crown and the variation b in the amount of crown in relation to the temperature variation are given by:
a=0.65 nm/deg. and
b=1.92 nm/deg.
If the range .+-.T of temperature variation is .+-.25 deg., the variation bT in the amount of crown is 48 nm and the variation abT in the amount of levitation is 31 nm. This indicates that in the magnetic disk apparatus using the conventional magnetic head assembly, the magnetic head assembly may possibly come into contact with the disk at high temperatures.
The amount in levitation of the magnetic head slider is influenced by a variance in quality of products, vibration, atmospheric pressure, etc. Taking these conditions into account, it is desirable that the distance between the magnetic head slider 3 and the top of the projection have an allowance of about 15% of the levitation amount H of the magnetic head slider 3, as compared to the variation amount of the levitation amount. About 15% of the levitation amount H of the magnetic head slider 3 is given by EQU 80.times.0.15=12 nm.
Under the circumstances, there is a demand for a measure to decrease the variation (the amount of crown variation per unit temperature) of the crown amount in relation to the temperature variation. In the above example, the following condition needs to be satisfied: EQU H(80 nm)-h(50 nm)-abT.gtoreq.12 nm.
That is, the following condition needs to be met: EQU b.ltoreq.1.11 nm/deg.
When the levitation amount of the regular magnetic head slider is decreased, the influence due to the variation in crown amount is great. Thus, the temperature-dependent variation in crown amount needs to be further decreased.